In most cases, temperature is the most important parameter to monitor when handling packed medicine, chemicals, and even food. The rate of many chemical reactions is typically exponential with temperature change (see Arrhenius equation). Thus the shelf life of a packaged good and the efficacy of the content can substantially change due to an exposure to high temperatures. In some cases, such as with packaged medicine, the exposure to high temperatures can render a drug ineffective, which can lead to dangerous situations.
It is therefore important to monitor and record the package temperature history and, at a minimum, be able to tell if a packaged good was exposed to a temperature that exceeds the maximum allowed temperature. While a variety of different techniques exist that could be used for monitoring packaging temperatures over time, the cost of the associated devices is typically too high as compared to the cost of the packaged goods to justify embedding such a device in each package. Accordingly, there is a need for a monitoring device with a cost so low that it does not substantially change the overall price of the packaged goods.
Another issue is that most monitoring devices require a power supply to maintain their operation. While the inclusion of a battery is possible, it raises the package cost, it limits the monitoring period to that of the battery life, and in very cold environments, where packages are sometimes stored, battery operation is limited. These issues make it more desirable to have a monitoring device that is passive, and does not require a power source for operation.
Thus, there is a need for a simple, low cost passive device to monitor materials in containers to assure that their content is safe and worthy of using.